Disc drives are used in workstations, laptops and personal computers to store large amounts of information in a readily accessible form. Typically, a disc drive includes a magnetic disc that is rotated at a constant high speed by a spindle motor. The disc surfaces are divided into a series of concentric data tracks that can store information as magnetic transitions on the disc surface.
A disc drive also includes a set of magnetic transducers that are used to either sense existing magnetic transitions during a read operation or to create new magnetic transitions during a write operation. Each magnetic transducer is mounted in a head, which in turn is mounted to a rotary actuator arm via a flexible element which can accommodate movement of the head during operation. The actuator arm serves to selectively position the head over a particular data track to either read data from the disc or to write data to the disc.
The actuator arm is driven by a voice coil motor. The magnetic transducers, mounted in the heads, are present at the ends of the arms which extend radially outward from a substantially cylindrical actuator body. This actuator body is moveably supported by a ball bearing assembly known as a pivot bearing or pivot bearing assembly. The actuator body is parallel with the axis of rotation of the discs. The magnetic transducers, therefore, move in a plane parallel to the discs surface.
The voice coil motor typically includes a coil which is mounted in the actuator arm at the end opposite the heads. This coil is permanently immersed in a magnetic field resulting from an array of permanent magnets which are mounted to the disc drive housing. Application of current to the coil creates an electromagnetic field which interacts with the permanent magnetic field, causing the coil to move relative to the permanent magnets. As the coil moves, the actuator arm also moves, causing the heads to move radially across the disc surface.
The heads literally fly on a wedge of air as the disc surface rotates in close proximity to the heads. A protective layer, for example a thin carbon overcoat, is widely used both on magnetic heads and on discs to protect against wear and damage. These thin carbon overcoats are about 50 to 200 Angstroms (.ANG.) in depth. The properties of these carbon overcoats have become increasingly important as the flight height, i.e., the distance between the heads and the spinning disc, continues to become smaller and the thickness of the carbon film becomes thinner. These changes are required to meet demands for ever-higher recording capacities. One of the important properties of thin carbon films is the film's resistivity as this property is directly related to the corrosion and degradation of the head/disc interface and therefore affects the tribological performance and reliability of the drive.
Resistivity can be used to monitor the carbon sputtering process used to create the thin carbon overcoat because the resistivity of the thin carbon film is highly sensitive to carbon composition and to any impurities that may be incorporated in the film.
However, measuring the resistivity of thin dielectric films can be difficult. One method previously used combines an electrometer with a two-point probe. The resistivity is obtained from the electrometer at a fixed voltage. Unfortunately, mechanical penetration of the carbon film is a problem as it is difficult to accurately control the load put on the disc through the probe. Another problem involves electrical penetration since very thin films tend to breakdown at voltages of as low as 5 volts. Therefore, the resistance values measured with this technique are usually characterized as having very large variations, perhaps in the range of 10 to 1000 times the mean value.
Another method previously used measure resistivity is a commercial technique known as the Four Point Probe. In this technique, four probes penetrate into the film to ensure good electrical contact. However, for very thin films on a metallic substrate, it is very difficult to control the penetration depth of the probe tips within the film. Frequently, the probes actually penetrate through the carbon layer to make direct contact with the metallic substrate. Because of this, large variations of measured resistance values also result.
Yet another method uses a mercury probe instead of a mechanical probe. This probe effectively prevents the mechanical penetration of the film described above. However, certain films suffer from mercury drop, which refers to a condition in which residual mercury remains on the surface of the film after the measurements. Consequently, this technique is limited to films having little or no affinity to mercury drop.
Consequently, a need remains for a simple and inexpensive technique for measuring the resistivity of thin dielectric films such as carbon overcoats. A need remains for a technique that is highly sensitive to impurity type and level, as well as being sensitive to small changes in film depth.